521 park avenue

Besides being an architectural gem, designed by William Alciphron Boring and completed in 1911, 521 Park Avenue is the rare classic pre-war Upper East Side building that’s also a condominium (it was converted in 1987). This sprawling duplex is the result of a high-floor two-unit combo. The resulting 3,000+ square-foot corner apartment has as much space and impressive pre-war detail as you’d expect from an address like this one.